Just like single-beam particle microscopes, multi-beam particle microscopes can be used to analyse objects on a microscopic scale. Images of an object that represent a surface of the object, for example, can be recorded using these particle microscopes. In this way, for example the structure of the surface can be analysed. While in a single-beam particle microscope a single particle beam of charged particles, such as electrons or ions, is used to analyse the object, in a multi-beam particle microscope, a multiplicity of particle beams are used for this purpose. The multiplicity of the particle beams, also referred to as a bundle, is directed onto the surface of the object at the same time, as a result of which a significantly larger area of the surface of the object can be scanned and analysed as compared to a single-beam particle microscope within the same period of time.
To this end, a multi-beam particle microscope includes numerous particle-optical components that produce and manipulate the multiplicity of particle beams. By way of example, a multi-beam particle microscope includes components which are configured to generate electric and/or magnetic fields in order to manipulate the particle beams, for example deflect them or change them in terms of their form. In particular, a multi-beam particle microscope can include a particle-optical lens which focuses each or individual ones of the particle beams. In addition, a multi-beam particle microscope can include a stigmator which can change the form of at least some of the particle beams, for example in order to correct a sub-optimal effect of another particle-optical component of the multi-beam particle microscope. In addition, further field generators may be provided, which can generate higher-order multipole fields, for example hexapole fields, so as to be able to compensate higher-order aberrations.
For the quality of an analysis of an object, it is generally desirable for the particle beams of the bundle to be focused on the surface of the object to be analysed. Similarly, it is generally advantageous for the quality of the analysis of the object if the particle beam spot that is illuminated by the particle beams on the surface of the object has a round shape, i.e. for the particle beams on the surface of the object to have an at best insignificant astigmatism. Arranging the surface of the object to be analysed within the focus of a particle beam is typically referred to as focusing. Focusing can typically be effected by manipulating the particle beams by correspondingly controlling particle-optical components of the multi-beam particle microscope or by correspondingly controlling the positioning of the object. Optimizing the particle beam spots that are illuminated by the particle beams, i.e. optimizing the form of the particle beams on the surface of the object, is typically referred to as stigmatizing. Stigmatizing is typically effected by way of manipulating the form of the particle beams, in particular by correspondingly controlling one or more stigmators and/or field generators of the multi-beam particle microscope.
Focusing, stigmatizing and correcting higher-order aberrations in manual operation of the multi-beam particle microscope by a user are time-consuming processes that involve a high level of experience of the user. Methods for automated focusing, stigmatizing and correcting of higher-order aberrations are known both in the field of single-beam particle microscopes and in the field of multi-beam particle microscopes. In some of these traditional methods, a plurality of particle-optical images of the same region of the surface of the object are recorded at different focusing or stigmatization settings so as to obtain herefrom an optimized setting for the focusing and stigmatization. By recording a plurality of images of the same region, these processes are time-consuming and reduce the throughput of the multi-beam particle microscope, i.e. the ratio of the area of the surface of the object to be analysed for a particular quality to a specified period of time.
In other traditional methods, in addition to the imaging particle beams serving for producing the image, further auxiliary beams are directed at the object which do not serve for producing the image, but only for setting the microscope in order to determine the quality of the focusing or stigmatization setting. The auxiliary beams are directed onto the region of the object to be imaged, i.e. the region of the surface of the object that is illuminated by the imaging particle beams, or on a region of the object that is arranged next to the region to be imaged. Charge carriers can accumulate in the region to be imaged in either case, i.e. the region to be imaged is contaminated. The charge that has accumulated in the region to be imaged generates electric fields in the region of the surface of the object that act on the imaging particle beams, as a result of which the particle beams are defocused or changed in terms of their form. These errors in focusing or stigmatization and also higher-order aberrations caused thereby deteriorate the quality of the images that are detected via the imaging particle beams.